1. Field of the Invention
The present invention relates to a telephone circuit and more particularly to a telephone circuit for detecting the loop current of a subscriber handset in a ground start mode and a normal mode which obviates the need for a separate TIP ground detector circuit and associated -48V supply to detect the subscriber handset off-hook status.
2. Description of the Prior Art
In known telephone systems, the on-hook/off-hook status of the subscriber handset is monitored. When the subscriber handset is off-hook, the off-hook status of the subscriber handset is used to enable the system to provide a dial tone to the handset as well as block incoming calls and provide a busy signal to any incoming callers. When the subscriber handset is on-hook the system is able to direct incoming calls to the subscriber handset as well as provide for ringing until the subscriber handset goes to an off-hook condition in which case the incoming call is connected to the subscriber handset.
FIG. 1 is an example of a known ground start telephone circuit, generally identified with the reference numeral 20. The dotted line 22 is used to show the division of circuitry between the subscriber handset and the central office. An outcall is typically initiated when the subscriber handset is off-hook. Initially a TIP ground detector circuit 24 (also known as a TIP bait circuit) is connected to a TIP conductor by way of a hook switch 26 that is closed when the subscriber handset is off-hook. The TIP ground detector 24 is connected to ground by way of a -48V power supply 28. If the TIP ground detector 24 detects that the TIP conductor has been grounded by way of a TIP ground switch 30 at the central office, an incoming call is pending and the outcall is aborted. Assuming there are no incoming calls, the switch 30 will be open.
Initially during a ground start mode of operation, the hook status of the subscriber handset during an outcall causes the RING conductor initially to be grounded by way of a hook switch 32 and a (optional) grounding resistor 34. Once the RING conductor is grounded at the subscriber handset, a RING status detector 36 at the central office detects the ground and closes the TIP ground switch 30. Once the TIP ground switch 30 is closed, the TIP ground detector 24 recognizes the ground and closes a switch 38 at the subscriber handset to enable the loop-hold circuit 40 to complete the loop between the subscriber handset and the central office. Once the loop-hold circuit 40 is enabled, the hook switches 26 and 32 are opened and a normal mode of operation is established. A loop current detector circuit 42 is used to measure loop current. After the loop current is detected by the loop current detector 42, the hook switches 26 and 32 are opened. The central office sends a dial tone and later establishes a voice connection with the subscriber handset.
As discussed above, the TIP ground detector circuit 24 is used to determine hook status to enable an outgoing subscriber handset to place an outgoing call or to receive an incoming call. As illustrated in FIG. 1, such a TIP ground detector circuit 24 is a separate circuit which requires a -48V supply. Unfortunately such a supply may not always be available. Moreover, the TIP ground detector circuitry 24 adds to the complexity of the overall system.
Circuitry has been developed which eliminates the need for a -48V supply, for example as described and illustrated in U.S. Pat. No. 4,282,408. In that system, a separate circuit is provided which includes a light emitting diode (LED) connected directly between the TIP and RING conductors of a subscriber handset circuit. The anode of the LED is connected to a power supply by way of a current limiting resistor while the cathode is connected to ground by way of a resistor. During an on-hook condition, the light emitting diode signals a phototransistor of the on-hook status of the subscriber telephone set. Once the subscriber telephone set goes off-hook, the circuit is configured so that the LED is shorted thereby eliminating the signal from the phototransistor. While such a circuit provides for the on-hook/off-hook status of the subscriber handset, such a system requires a separate circuit and thus adds to the overall complexity and cost of the system.